1. Field of the Invention
The invention relates generally to electronic circuitry. More specifically, the invention relates a method for reducing the noise associated with a clock signal in a flip-flop based circuit.
2. Background Art
In all microprocessor-based systems, including computers, the clock circuit is a critical component. The clock circuit generates a clock signal that is a steady stream of timing pulses that synchronize and control the timing of every operation of the system. FIG. 1 shows a prior art diagram of an ideal clock signal 10. An entire clock cycle 12 includes a rising or leading edge 14 and a falling or trailing edge 16. These edges 14, 16 define the transition between the low and high value of the signal.
FIG. 2 shows a block diagram of a prior art local clock signal distribution system. The clock signal 30a is input to a clock header 32 which serves to buffer the clock signal. From the header 32, the clock signal 30b is input to an edge-triggered flip flop 34 (xe2x80x9cflip-flopxe2x80x9d) where it serves to trigger the flip-flop. A flip-flop is a memory device that is commonly used in integrated circuits. It is dependent upon a clock signal to initiate its function. Flip-flops generally take input data and distribute output data on the rising edge of a clock signal. However, a flip-flop could be configured to work on the falling edge of a clock signal.
FIG. 3 shows a digital logic schematic of the prior art local clock signal distribution system as shown in FIG. 2. The clock signal 30a is input to the clock header 32. The clock header 32 includes a NAND gate 36 and an inverter 38a. Once inside the clock header 32, the clock signal 30a is one of the inputs to the NAND gate 36. The other NAND input 42 is a signal that is HIGH so that the gate 36 simply inverts the value of the clock signal 30a. The NAND input 42 is switched to LOW to turn off the clock header 32 if needed. Next, the signal 30a passes through the inverter 38a which inverts the signal back to its original value. The clock signal 30b then passes from the clock header 32 to the flip-flop 34. Once in the flip-flop 34, the signal 30b is split into two paths. The first path passes through one inverter 38b, and the second path passes through two consecutive inverters 38c and 38d. Each path feeds into the internal circuitry of the flip-flop 40 along with the DATA_IN 44 and DATA_OUT 46 paths of the flip-flop 34.
Clock noise problems on the system power grid are usually caused by the large amount of current that is used in clock signal distribution. This current comes from the switching transistors that control the clock signal. As these transistors switch states, the current noise spikes onto the power grid due to the current demand or xe2x80x9ccurrent drawxe2x80x9dof the switching transistors. These high current demands cause noise in the system voltage supply due to voltage (IR) drops and inherent system inductance (L di/dt). A clock signal distribution circuit uses a significant amount of current in a short amount of time because the spikes occur twice per clock cycle: once on the current draw of the leading edge and once on the current draw of the falling edge of the signal. This puts the noise at a very high frequency (2xc3x97the clock frequency). This noise can cause missed timing if the clock signal voltage is too low or component failure if the clock signal voltage is too high. The noise can even escape xe2x80x9coff the chipxe2x80x9d and affect the other components of the system.
FIG. 4 shows a graph of current draw during a clock cycle period of the prior art embodiment shown in FIG. 3. The flip-flop of this embodiment is triggered on the rising edge of a clock signal. The value xe2x80x9cIxe2x80x9d 35 represents the full value of a current draw. The value xe2x80x9c{fraction (3/4+L )} Ixe2x80x9d 37 represents 75% of the full value while the value xe2x80x9c{fraction (1/2+L )} Ixe2x80x9d 39 represents 50% of the full value. The first current draw 41 of the graph represents the draw that results from the leading edge of a clock cycle (at clock cycle=0). The second current draw 43 represents the draw that results from the falling edge of the clock cycle (at clock cycle=t/2). As shown, the leading edge draw 41 is the full value of current draw. The trailing edge draw 43 is about half the value of the leading edge draw 41. In this example, the first draw 41 is larger than the second draw 43 because all of the flip-flop""s change on the rising edge of the clock signal. Since none of the flip-flops change on the falling edge, the second draw 43 is smaller.
A common technique to alleviate noise is adding additional power to the grid. This power is added upon sensing a voltage drop due to noise. However, such techniques only respond to noise at a much lower frequency than clock noise and also respond only to a certain threshold of noise. Consequently, a need exists for a technique that generates a response to clock noise at a synchronized frequency with the clock noise itself.
In some aspects, the invention relates to an apparatus for reducing noise of a clock signal for a flip-flop based circuit, comprising: a charge control circuit that initiates storing a charge upon receipt of a first pre-determined signal; and a dump control circuit that initiates dumping the charge onto a system power grid upon receipt of a second predetermined signal.
In another aspect, the invention relates to an apparatus for reducing the noise of a clock signal for a flip-flop based circuit, comprising: means for initiating a charge control circuit that stores a charge upon receipt of a first pre-determined signal; and means for initiating a dump control circuit that dumps the charge onto a system power grid upon receipt of a second pre-determined signal.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.